Why Does the Road Rise in Winter? Ice Pulls Water Towards Itself

Water expands by about nine per cent when it freezes, which sounds like enough to explain a lifted road until the arithmetic is done. A soil holding thirty per cent water by volume would lift by under three per cent of its depth if that water simply froze in place, which for the top metre of ground is under three centimetres. Measured lifting regularly exceeds ten centimetres and has been recorded at far more. Something else must be supplying the volume, and what is happening is that water is arriving from elsewhere and freezing as an addition rather than the existing water expanding where it sits.

The process builds layers of clear ice within the soil:

• •Freezing begins at a depth where the cold has reached
• •Ice forming there lowers the pressure of the remaining unfrozen water
• •That draws liquid water upward from the warmer soil below
• •The arriving water freezes onto the ice already there
• •The lens thickens and lifts everything above it bodily
• •The supply continues as long as water and cold are both available

Three things must coincide, and removing any one stops the process, which is the basis of every engineering remedy. There must be freezing temperatures reaching into the ground. There must be a supply of water within reach, meaning a water table close enough for it to be drawn up. And the soil must have pores of roughly the right size, which is the condition people find surprising. Coarse gravel has pores too large to hold water against gravity and draw it upward, so it does not lift. Dense clay holds water tightly but transmits it far too slowly to feed a growing lens. Silt sits exactly in between and is the worst material there is, which is why silty subgrades are dug out and replaced.

Every remedy removes one of the three requirements, and knowing which one is being attacked makes the measures legible. Founding below the depth to which frost reaches removes the cold, which is why building codes specify a minimum foundation depth that varies by region and why that figure is much larger in cold countries. Replacing a susceptible subgrade with clean coarse gravel removes the pore sizes that draw water. Lowering the water table with drainage removes the supply. Insulating horizontally outward from a foundation keeps ground heat in and prevents the frost reaching under it, which allows shallower footings. And a layer of rigid insulation under a road does the same for pavement.

The damage is expensive and familiar in cold regions, with roads cracking as they lift unevenly, foundations and fence posts pushed upward, and buried pipes displaced. Thawing is frequently worse than freezing, since the melting lenses release far more water than the soil originally held while the ground below is still frozen and cannot drain it, leaving a saturated weak layer that fails under traffic, which is why spring load restrictions exist. The same process also builds landforms, sorting stones into the polygons and stripes seen on arctic and mountain ground, raising the small mounds found in tundra, and lifting buried stones to the surface of fields year after year.

Freezing water in place could lift the ground only a few centimetres, and the much larger movements observed come from liquid water being drawn upward to feed growing lenses of clear ice. Cold, a water supply and pores of the right size must all be present, and silt is the worst material because gravel drains and clay transmits too slowly. Thaw is worse than frost, since melting lenses saturate ground that cannot drain.